A CQI (Channel Quality Indicator), which is used for, for example, selecting an MCS (Modulation and Coding Scheme), is reported from UE (User Equipment) to node B. If the UE reports CQIs frequently, although the accuracy of MCS selection improves at the node B, the uplink channel becomes busy. For this reason, studies are underway to reduce the total number of CQIs.
Non-patent document 1 discloses a method of layering RBs (Resource Blocks) and reporting the maximum CQI on a per layered RB basis. FIG. 1 shows layered RBs. FIG. 1 shows a case where twelve RBs are each provided and classified as four layers. Layer 1 (n=1) groups all the RBs into one group, layer 2 (n=2) equally groups the RBs into three every four RBs. Further, layer 3 (n=3) equally groups the RBs into six every two RBs, and layer 4 (n=4) groups the RBs every one RB (i.e. in RB units).
The CQI selection method in the case where RBs are layered, will be explained. First, in layer 1, the CQI is selected based on an average SIR of all RBs. In layer 2, the CQI is selected based on the maximum average SIR among the average SIRs every four RBs, and, similarly, in layer 3, the CQI is selected based on the maximum average SIR among the average SIRs every two RBs. In layer 4, the CQI is selected based on the maximum SIR among the SIRs per RB.
FIG. 2 shows channel conditions and reporting CQIs on a per layer basis shown in FIG. 1. In FIG. 2, the bold solid line shows the CQI level when the CQI is reported per RB, the thin solid line (straight line) shows the CQI level reported in layer 1 (n=1). Further, the dotted line shows the CQI level reported in layer 2 (n=2), the dash-dotted line shows the CQI level reported in layer 3 (n=3) and the dash-two dot line shows the CQI level reported in layer 4.
When there are twelve RBs in total as shown in FIG. 1, and a five bit CQI is reported on a per RB basis, sixty bits are required. However, when RBs are layered and CQIs are reported on a per layer basis, the number of required bits can be reduced to twenty bits.
Although with the above example the average SIR or the maximum SIR value are used as a CQI selection reference, the minimum value may also be used and the maximum and minimum values may also be used.
As described above, CQIs are reported on a per layer basis, and, at the receiving side, CQIs of higher layers are overwritten with CQIs of lower layers and are identified as CQI per RB.
Non-patent Document 1: 3GPP TSG RAN1 #44, R1-060381, “Frequency Domain Scheduling for E-UTRA,” Denver, US, February 13-17, 2006